Wave guides have become familiar means to transmit high radio frequency signals, especially in the microwave region. More recent developments have made wave guides for the transmission of light rather commonplace. An optical fiber is a wave guide in which light is propagated by total internal reflection at the fiber boundaries. For the purpose of this application, "light" will refer to electromagnetic radiation in the ultraviolet, visible, and infrared ranges of the electromagnetic spectrum, which are approximately 200-400 nm, 400-800 nm, and 800-300,000 nm, respectively, extending through the far infrared range. The greatest use of optical fibers has been in communication and data transmission systems where light waves of a narrow wavelength are used as carriers via pulse or frequency modulation to transmit information. A less common but increasingly important application of optical fibers is for the transmission of analog information from a sensor to a remotely located detector which measures the intensity of the transmitted light over a range of wavelengths within the spectrum of light. For the purpose of this application the spectral range of greatest interest is that spanning the ultraviolet (ca. 200-400 nm), visible (ca. 400-800 nm) and near infrared (ca. 800-2500 nm).
The measurement of, for example, digital information differs significantly from that of analog information and imposes different requirements. Where digital information is transmitted along an optical fiber one is interested only in whether or not a signal is present, or more accurately whether light of a particular frequency is present at an intensity above some threshold value. Where analog information is transmitted along optical fibers one is interested in the absolute intensity of the signal at each wavelength of some extended portion of the light spectrum. Thus it becomes clear that where accurate transmission of analog information along an optical fiber is required it is necessary that both the wavelength and intensity of the transmitted light be preserved, that is, one can tolerate neither wavelength shifts nor intensity variation along the transmission path.
The principles of optical fibers are too well known to require extended discussion here. See, for example, "Optical Fiber Communications", B. K. Tariyal and A. H. Cherin, Encyclopedia of Physical Science and Technology, Vol. 9, pp 605-629 (1987); "Optical Fibers, Drawing and Coating", L. L. Blyler, Jr. and F. V. DiMarcello, ibid., pp 647-57. In brief, optical fibers have a core of plastic, glass, silica or other glassy transparent material with an outer, concentric layer called cladding which has a refractive index lower than the core. Where light injected into the core strikes the core-cladding interface at an angle of incidence greater than the critical angle there is total reflection, and since the angle of incidence equals the angle of reflection it follows that light will zigzag or spiral along the length of the core. Although in theory there should be no light loss, in practice attenuation occurs along the optical fiber because of the absorption by impurities within the core and because of scattering arising mainly from fiber imperfections such as non-uniform core diameter, bends in the fiber, and discontinuities at the core-cladding interface.
Optical fibers per se are delicate and fragile, and generally need to be protected by being sheathed with several concentric layers. In a variant of interest here the core of the optical fiber is coated during the drawing process with a thin layer of a tough polymer, such as a polyimide, to protect the delicate surface from scratching and marring, and to prevent microfracture. This is followed by another concentric layer of an elastic polymer, such as silicones, thermoplastic rubber compounds, urethanes or acrylates. Yet other coatings may be applied as protection from physical and chemical damage. It also should be noted that in another variant the cladding itself may be an elastic polymer. Of special interest is the case where the optical fiber comprises concentric layers of a glassy core of refractive index n, a glassy cladding of refractive index less than n, a polyimide coating, and a silicone coating. It needs to be emphasized that even though such an optical fiber is of special interest to us, our invention is not limited to such an optical fiber but is instead applicable to optical fibers generally.
We recently observed spurious signals in light transmitted along optical fibers under two quite different circumstances. In one case the intensity of light transmission varied with the intensity of ambient light external to the optical fiber. Thus, the light intensity measured at different wavelengths at the exit of an optical fiber varied with the intensity of external light. This implied that there was a significant amount of extraneous light from a source external to the fiber entering the core through the cladding along the length of the optical fiber, contrary to expectations. By "extraneous light" is meant light inserted into the core of an optical fiber through the cladding, in contradistinction to light injected directly into the core.
The second circumstance of spurious light transmission was noticed in an optical fiber having bends along its length and was manifested as selective attenuation at certain wavelengths. Further investigation showed that the wavelengths whose intensity were reduced corresponded to spectral absorption bands of a coating for the fiber. Evidently light was not totally reflected at the core-cladding interface at bends in the fiber but was reflected at the surface of other coatings acting as a secondary cladding. Thus light traversed the core-(primary)cladding interface, through one or more coatings external to the primary cladding where selective absorption occurred, was reflected at the interface with the secondary cladding back through the coatings it had already traversed where absorption occurred for a second time, and finally entered the core once more. In summary, light escaped the core, travelled through one or more layers of coatings, there to be selectively absorbed, and was reflected back into the core. Thus, light reentering the core corresponded, at least roughly, to the "absorption spectrum" of the traversed coatings and led to selective signal attenuation.
Once the nature of these problems was determined both were susceptible to a common solution. If any coating contained one or more components which efficiently absorbed the light entering the coating the problem could be expected to be effectively solved for the case of incident light. If the coating containing the light-absorbing component was placed between the primary cladding and the secondary cladding both problems would be solved. In fact, that turned out to be correct in the foregoing cases.